Methods of making and using a chemical-mechanical polishing slurry that reduces wafer defects

ABSTRACT

A method of making a chemical-mechanical polishing slurry includes mixing a ferric salt oxidizer with a solution to produce a mixture with a dissolved ferric salt oxidizer, filtering the mixture to remove most preexisting particles therein that exceed a selected particle size, adding a suspension agent to the mixture, and adding abrasive particles to the mixture after filtering the mixture. Advantageously, when polishing occurs, scratching by the preexisting particles is dramatically reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to polishing slurry, and more particularlyto a chemical-mechanical polishing slurry that reduces wafer defects andits method of making.

2. Description of Related Art

In the manufacture of integrated circuits, the planarization ofsemiconductor wafers is becoming increasingly important as the number oflayers used to form integrated circuits increases. For instance,metallization layers formed to provide interconnects between variousdevices may result in nonuniform surfaces. The surface nonuniformitiesmay interfere with the optical resolution of subsequent lithographicsteps, leading to difficulty with printing high resolution patterns. Thesurface nonuniformities may also interfere with step coverage ofsubsequently deposited metal layers and possibly cause open or shortedcircuits.

Various techniques have been developed to planarize the top surface of asemiconductor wafer. One such approach involves polishing the waferusing a polishing slurry that includes abrasive particles mixed in asuspension agent. With this approach, the wafer is mounted in a waferholder, a polishing pad has its polishing surface coated with theslurry, the pad and the wafer are rotated such that the wafer provides aplanetary motion with respect to the pad, the polishing surface ispressed against an exposed surface of the wafer, and the slurry is usedas a hydrodynamic layer between the polishing surface and the wafer. Thepolishing erodes the wafer surface, and the process continues until thewafer topography is largely flattened.

In chemical-mechanical polishing (CMP), the abrasive particles providefriction while oxidants and/or etchants cause a chemical reaction at thewafer surface. Additives can also be added to enhance the removal rate,uniformity, selectivity, etc., and dilution by deionized water is alsopracticed.

CMP is becoming a preferred method of planarizing tungsteninterconnects, vias and contacts. Tungsten CMP slurries typicallyinclude abrasive particles such as alumina, a ferric salt oxidizer suchas ferric nitrate, a suspension agent such as propylene glycol, anddeionized water. With proper process parameters, CMP tungsten processinghas shown significantly improved process windows and defect levels overstandard tungsten dry etching. One significant advantage of CMP tungstenprocessing is that it has a highly selective polish rate for tungsten ascompared to the dielectric. This selectivity allows for over-polishingwhile still achieving a flat tungsten stud. When overetching occursusing dry etching, the contact or via becomes further recessed, whichcreates a serious disadvantage since overetching is frequently requiredto remove defects.

The advantages of CMP, however, can be offset by the creation ofsignificant defects during polishing, such as scratches. The prior artteaches that scratching can be controlled by the proper manufacturing,size control and filtering of the abrasive particles. The prior art alsoteaches that the proper mixing sequence of the abrasive particles withthe suspension agent leads to lower defects.

Unfortunately, for various reasons, prior CMP slurries have not been aseffective as needed. In particular, deep or wide scratch defects in thepolished surface continue to cause problems. This may arise sinceconventional slurry filtering tends to remove only those particles thatare significantly larger than most of the abrasive particles. Therefore,a need exists for an improved CMP slurry that reduces scratchingdefects.

SUMMARY OF THE INVENTION

An object of the invention is to provide a CMP slurry which enablesplanarization of a polished layer and reduces scratching detects. Theseobjects are achieved by filtering a solution with a dissolved oxidizerbefore adding the abrasive particles to the mixture, thereby removing asubstantial amount of preexisting particles in the solution.

In accordance with one aspect of the invention, a method of making a CMPslurry includes mixing a ferric salt oxidizer with a solution to producea mixture with a dissolved ferric salt oxidizer, filtering the mixtureto remove most preexisting particles therein that exceed a selectedparticle size, adding a suspension agent to the mixture, and addingabrasive particles to the mixture after filtering the mixture.Advantageously, when polishing occurs, scratching by the preexistingparticles is dramatically reduced due to the filtering operation.

These and other objects, features and advantages of the invention willbe further described and more readily apparent from a review of thedetailed description of the preferred embodiments which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the preferred embodiments can bestbe understood when read in conjunction with the following drawings, inwhich:

FIG. 1 is a chart of particle size versus particle count for filteredand unfiltered ferric nitrate solutions;

FIG. 2 is a chart of total particles for samples of filtered andunfiltered ferric nitrate solutions;

FIG. 3 is an EDX graph of amorphous white particles filtered from aferric nitrate solution;

FIG. 4 is an FTIR graph of amorphous white particles filtered from aferric nitrate solution;

FIG. 5 is an EDX graph of amber flakes filtered from a ferric nitratesolution;

FIG. 6 is an FTIR graph of amber flakes filtered from a ferric nitratesolution;

FIG. 7 is an EDX graph of a survey scan of a filter used for a ferricnitrate solution; and

FIG. 8 is an FTIR graph of a survey scan of a filter used for a ferricnitrate solution

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Commercially available CMP equipment and slurries are currentlyavailable for planarization of integrated circuits with tungsten viasthrough silicon dioxide layers. The commercially available slurries,however, exhibit problems such as high scratch counts. Our slurrysubstantially addresses and reduces these problems.

We have discovered that preexisting particles in slurries cansignificantly contribute to scratching. As used herein, "preexistingparticles" generally refer to unwanted particles that exist in a mixtureof an oxidizing agent and a solution before the desired abrasiveparticles (such as alumina) are added. We believe the preexistingparticles include undissolved oxidizing agent, contaminants and/orreaction products formed in the mixture. We specifically believe, forinstance, that a contaminant is dust, and a reaction product of a ferricnitrate oxidizer is an organic nitro compound. Moreover, as the slurryages, the preexisting particles tend to grow and/or coalesce. As aresult, when polishing occurs, the preexisting particles can causesubstantial wafer damage.

In accordance with one aspect of the invention, a slurry is prepared bymixing a ferric salt oxidizer with a solution to produce a mixture witha dissolved ferric salt oxidizer, filtering the mixture to remove asubstantial amount of the preexisting particles therein, adding asuspension agent to the mixture, and adding abrasive particles to themixture after filtering the mixture.

Preferably, the filtering removes most of the preexisting particles thatexceed a particle size of about 0.1 microns, most of the abrasiveparticles have a particle size in the range of about 0.2 to 0.7 microns,and the slurry is used for polishing within one day of the filteringoperation. In this manner, the preexisting particles that remain in theslurry exhibit relatively little growth or coalescence before polishingoccurs. Furthermore, as polishing occurs, the preexisting particlescause very little scratching since most of the abrasive particles have afar larger particle size than that of the preexisting particles.

Preferably, the ferric salt oxidizer and the solution are thoroughlymixed before filtering occurs so that essentially all of the ferric saltoxidizer is dissolved. However, if undissolved ferric salt oxidizerremains, the filtering removes most of these particles which exceed aselected particle size.

Preferably, the suspension agent and the abrasive particles arethoroughly pre-mixed so that the suspension agent wets the abrasiveparticles. It is also preferred that the pre-mixed suspension agent andabrasive particles are added to the mixture immediately after filteringthe mixture, especially if polishing occurs several days after thefiltering operation, and that the suspension agent inhibits the growthand/or coalescence of the preexisting particles.

Finally, it is preferred that most of the preexisting particles areFe-containing particles (such as undissolved ferric salt oxidizer), thatthe preexisting particles have a different composition than the abrasiveparticles, and that none of the abrasive particles (or particles withthe same composition) are in the mixture before filtering the mixture.

Other sequences can be used. For instance, the suspension agent canadded to the mixture before filtering the mixture. Likewise, thesuspension agent can be added to the mixture after filtering themixture, and then the abrasive particles can be added to the mixture. Itis essential, however, that the abrasive particles be added to mixtureafter filtering the mixture.

The ferric salt oxidizer can be formulated from suitable Fe compounds,such as ferric nitrate (Fe(NO₃)₃.9H₂ O), ferric chloride hexahydrate(FeCl₃.6H₂ O), ferric sulfate pentahydrate (Fe₂ (SO₄)₃.5H₂ O) and ferricammonium sulfate dodecahydrate (FeNH₄ (SO₄)₂.12H₂ O).

The preferred solution in which the ferric salt oxidizer is initiallymixed is ultra-pure, deionized water.

The suspension agent (also referred to as a dispersion agent) ispreferably an aqueous surfactant that improves the colloidal behavior ofthe abrasive particles in deionized water, and inhibits the growthand/or coalescence of the preexisting particles. For instance, thesuspension agent can be a commercially available aqueous mixture ofpropylene glycol and methyl paraben. The suspension agent can beformulated from the following classes:

1) glycols such as ethylene glycol, propylene glycol and glycerol;

2) polyethers such as polyethylene glycol;

3) aliphatic polyethers; and

4) akoxylated alkyphenols.

The abrasive particles can be any of the commonly used abrasives such asalumina (Al₂ O₃), silicon carbide (SiC), silicon dioxide (SiO₂), ceria(CeO₂) and silicon nitride (Si₃ N₄).

The resultant slurry is well-suited for CMP polishing a predominantlytungsten layer during the fabrication of an integrated circuit device.We believe that CMP of tungsten films takes place by a chemicaloxidation of the tungsten surface with a suitable ferric salt oxidizingagent in an aqueous solution, followed by mechanical abrasion of themore brittle metal oxide which has formed on the surface by the solidabrasive particles present in the aqueous suspension. Both the oxidationand the abrasion continue simultaneously and continuously. The reactionfor tungsten by the ferric ion is

    W+6Fe.sup.+++ +3H.sub.2 O→WO.sub.3 +6Fe.sup.++ +6H.sup.+

and occurs in an acid solution.

Several CMP experiments (Experiments 1, 2 and 3) as set forth below wereperformed to determine the affects of filtering a ferric nitratesolution on particle counts and scratches of a subsequently polishedwafer. A chemical analysis Experiment 4) of filtered material from agedferric nitrate solution was also performed as set forth below.

Experiment 1

In this experiment, the affects of filtering a ferric nitrate solutionwere investigated. An unfiltered ferric nitrate solution with 0.33Mconcentration was prepared by adding 500 grams of ferric nitratecrystals to 1 gallon of ultra-pure water (UPW). The solution wasagitated slightly by either shaking the bottle or with the aid of asmall mixing blade to ensure all the ferric nitrate was dissolved. Afiltered ferric nitrate solution was initially prepared the same way asthe unfiltered solution, and then filtered through a 0.1 micron filter.A surfactant, which was not mixed with the other solutions, was acommercially available aqueous mixture of propylene glycol and methylparaben from Universal Photonics, Inc., sold under the trade name"Everflo White".

Particle tests were performed by pouring about 2500 ml of each solutiononto a polishing pad while running a particle monitor wafer loaded on apolisher. The order of particle tests was as follows:

1. UPW (to verify the polisher was clean)

2. Surfactant

3. Filtered ferric nitrate solution (tested immediately after filtering)

4. Unfiltered ferric nitrate solution

The metrology tool measured defects, which included particles andscratches. The number of defects added in the 0.2 to 0.3 micron range,0.3 to 0.4 micron range, and above 0.4 micron were measured, and thensummed to provide total defects added, as listed below in Table 1. Anegative number indicates the removal of particles from the initialcount on the particle monitor wafer.

                                      TABLE 1    __________________________________________________________________________               Defects Added                      Defects Added                             Defects Added               0.2 to 0.3                      0.3 to 0.4                             Above 0.4                                    Total Defects    Solution   Micron Micron Micron Added    __________________________________________________________________________    UPW        -2     -7     -3     -12    Surfactant 12     0      1      13    Filtered Ferric Nitrate               11     0      1      12    Unfiltered Ferric Nitrate               1767   91     171    2029    __________________________________________________________________________

Table 1 indicates that filtering the ferric nitrate solution drasticallyreduced defects on a wafer polished immediately after the filteringoperation.

The experimental procedure and equipment for Experiment 1 were asfollows:

Equipment: IPEC 472 Avanti Polisher.

Wafer Carrier: Standard design.

Polish Pad: Industry standard.

Particle Monitor: 6000 Å of plasma TEOS (CVD deposited) on prime siliconwafer.

Dummy Wafer: 20,000 Å of plasma TEOS (CVD deposited) on prime siliconwafer.

Metrology Tool: Tencor 6400 Surfscan.

Pad Condition: New pad prior to particle tests.

Process Cycle: Set time of 30 sec per n, 5 psi 25 rpm carrier, 100 rpmplaten.

Loading Sequence: Ran 1 dummy wafer under followed by the particlemonitor wafer for each solution tested.

Experiment 2

In this experiment, the affects of aging unfiltered ferric nitratesolution, filtered ferric nitrate solution, and filtered ferric nitratesolution mixed with a surfactant were investigated. A ferric nitratesolution with a 0.98M concentration was prepared by adding 5 kg offerric nitrate crystals to 3.33 gallons of UPW. The solution wasagitated with the aid of a small mixing blade to ensure all the ferricnitrate was dissolved. The solution was then divided three ways. A firstgallon of solution was filtered through a 0.1 micron filter and thenmixed immediately with 1 gallon of surfactant (Everflo White), bottledand stored. A second gallon of solution was filtered through a 0.1micron filter, bottled, and stored. A third gallon of the solution wasleft unfiltered and stored in a bottle.

The solutions were particle tested after aging for 14 days. The particletests were performed in a similar manner as the previous experiment. Theorder the particle tests was as follows:

1. UPW (to verify the polisher was clean)

2. Surfactant and filtered ferric nitrate solution

3. Filtered ferric nitrate solution

4. Unfiltered ferric nitrate solution

The results of the particle tests are listed below in Table 2.

                                      TABLE 2    __________________________________________________________________________               Defects Added                      Defects Added                             Defects Added               0.2 to 0.3                      0.3 to 0.4                             Above 0.4                                    Total Defects    Solution   Micron Micron Micron Added    __________________________________________________________________________    UPW        17     2      -8     11    Surfactant and Filtered               120    3      14     137    Ferric Nitrate    Filtered Ferric Nitrate               281    5      -4     282    Unfiltered Ferric Nitrate               4810   509    922    6241    __________________________________________________________________________

Table 2 indicates that aging the ferric nitrate solutions led to farmore defects. In addition, the shelf life was improved by filtering theferric nitrate, and further improved by mixing the filtered ferricnitrate solution with the surfactant immediately after the filteringoperation.

The experimental procedure and equipment for Experiment 2 were asfollows:

Equipment: IPEC 472 Avanti Polisher

Wafer Carrier: Standard design.

Polish Pad: Industry standard.

Particle Monitor: 6000 Å of plasma TEOS (CVD deposited) on prime siliconwafer.

Dummy Wafer: 20,000 Å of plasma TEOS (CVD deposited) on prime siliconwafer.

Metrology Tool: Tencor 6400 Surfscan.

Pad Condition: New pad prior to particle tests.

Process Cycle: Set time of 30 sec per run, 5 psi, 25 rpm carrier, 100rpm platen.

Loading Sequence: Ran 1 dummy wafer under followed by the particlemonitor wafer for each solution tested.

Experiment 3

In this experiment, the affects of aging filtered and unfiltered ferricnitrate solutions were investigated by liquid particle counting. Thefiltered and unfiltered ferric nitrate solutions used in Experiment 2(without the surfactant) were diluted to a concentration of about 0.002Mby adding about 1 part solution to about 500 parts UPW. Liquid particlecounting was performed on the diluted filtered and unfiltered ferricnitrate solutions after 4 days of aging, and on the diluted filteredferric nitrate solution after 10 days of aging.

FIGS. 1 and 2 show the results of the liquid particle counting. FIG. 1is a chart of particle count versus particle size, and FIG. 2 is a chartof total particles for the samples used. FIGS. 1 and 2 demonstrate thatfiltering the ferric nitrate leads to dramatic improvements in shelflife as compared to unfiltered ferric nitrate.

The experimental procedure and equipment for Experiment 3 were asfollows:

Equipment: CLS-700 liquid particle counter manufactured by ParticleMonitoring System and an NEC laptop computer.

Purge Gas: N₂.

Specimen Container: Squeeze bottle.

Measured Particle Size: 0.2 to 2.0 microns.

Sampling Rate: Standard.

Experiment 4

In this experiment, the composition of the preexisting particles in agedferric nitrate solution was investigated. An unfiltered ferric nitratesolution of 0.6M concentration was prepared by adding 460 grams offerric nitrate crystals to 0.5 gallons of UPW in a polypropylene bottle.The solution was shaken for about 1 minute, stored for 45 days, and thenfiltered through a 0.8 micron gold-coated membrane filter. After airdrying, the filter was examined by optical and SEM microscopy toidentify characteristic particles. Many of the particles were amorphouswhite particles resembling polymers, while several others were amberflakes. Few smaller particles (i.e., 1-5 micron diameters) wereobserved.

The amorphous white particles, the amber flakes, and a survey scan ofthe filter (in an area free from larger particles) were analyzed by EDXspectroscopy to determine elemental composition and by FTIR spectroscopyto identify chemical structure. The results are listed below in Table 3.

                  TABLE 3    ______________________________________               Elemental   Chemical    Particle   Composition Structure    ______________________________________    Amorphous White               Major: C    polyethylene and polypropylene    Particles  Minor: O, Si, Fe                           and inorganic silicates               Trace: Al, Cl, Ti    Amber Flakes               Major: O, Fe, Si                           polydimethylsiloxane (silicone)               Minor: C, Cl                           and polymer and ferric nitrate               Trace: Al, S                           and water of hydration and                           organic nitro compound    Survey Scan               Major: Au (filter)                           polydimethylsiloxane (silicone)               Minor: C, O and polymer and inorganic               Trace: Al, Si                           silicates and trace polyethylene    ______________________________________

FIG. 3 shows the EDX graph for the amorphous white particles, FIG. 4shows the FTIR graph for the amorphous white particles, FIG. 5 shows theEDX graph for the amber flakes, FIG. 6 shows the FTIR graph for theamber flakes, FIG. 7 shows the EDX graph for the survey scan, and FIG. 8shows the FTIR graph for the survey scan.

The EDX and FTIR analysis indicate that most of the filtered particleswere polymeric. In the amorphous white particles, the polyolefins(polyethylene and polypropylene) probably came from the polypropylenebottle used to mix and store the solution, and the inorganic silicatesprobably came from airborne dust but might have been added as fillers topolymer products. Thus, the amorphous white particles appear to becontaminants that can be substantially reduced or eliminated bypreparing the solution in a cleaner environment. In the amber flakes,the ferric nitrate appears to be undissolved crystals, the organic nitrocompound appears to be a reaction product formed in the solution, andthe silicone polymers probably came from sealants, lubricants, defoamingagents, o-rings or gaskets.

FIGS. 1 and 2 demonstrate that the preexisting particles get larger asthe solution ages. (That is, the median preexisting particle sizeincreases, although it is entirely possible that many of the preexistingparticles do not get larger.) The exact mechanism by which thepreexisting particles grow and/or coalesce is not presently understood.

Accordingly, our slurry includes abrasive particles, most of which arelarger than a selected particle size, an oxidizer, a suspension agent,and preexisting particles, having been filtered, so that most aresmaller than the selected particle size. Although the preferred oxidizeris a ferric salt, other oxidizers such as ammonium persulfate aresuitable. The slurry can be used to polish various metals such astungsten, aluminum and copper.

The slurry is well-suited for use in a polishing system for polishing asemiconductor wafer that includes a polishing pad with a polishingsurface, a rotatable platen for removably securing the polishing pad, arotatable wafer holder for removably securing a wafer such that thewafer can be pressed against the polishing surface, and a dispenser fordispensing the slurry onto the polishing surface. A method of polishinga semiconductor wafer with the slurry includes providing a polishing padwith a polishing surface, mounting a semiconductor wafer on a waferholder, rotating the polishing surface, introducing the slurry onto thepolishing surface, and planarizing the wafer using the rotatingpolishing surface and the slurry.

Other variations and modifications of the embodiments disclosed hereinmay be made based on the description set forth herein, without departingfrom the scope and spirit of the invention as set forth in the followingclaims.

What is claimed is:
 1. A method of polishing a metal layer during thefabrication of an integrated circuit device, comprising the followingsteps:providing a chemical-mechanical polishing slurry, including:mixingan oxidizer with a solution to produce a mixture with a dissolvedoxidizer; filtering the mixture to remove most particles therein thatexceed a selected particle size; adding a suspension agent to themixture; and adding abrasive particles to the mixture after filteringthe mixture, wherein most abrasive particles exceed the selectedparticle size; and then polishing the metal layer with the slurry. 2.The method of claim 1, wherein the selected particle size is at mostabout 0.1 microns.
 3. The method of claim 1, whereinthe abrasiveparticles are selected from the group consisting of Al₂ O₃, SiC, SiO₂,CeO2 and Si₃ N₄ ; the oxidizer is a ferric salt oxidizer selected fromthe group consisting of Fe(NO₃)₃.9H₂ O, FeCl₃.6H₂ O, Fe₂ (SO₄)₃.5H₂ Oand FeNH₄ (SO₄)₂.12H₂ O; the suspension agent is an aqueous surfactant;the solution is water; and the metal layer is predominantly tungsten. 4.The method of claim 3, wherein the abrasive particles are Al₂ O₃, andthe ferric salt oxidizer is Fe(NO₃)₃.9H₂ O.
 5. A method of polishing asemiconductor wafer, comprising:providing a polishing pad with apolishing surface; mounting a semiconductor wafer on a wafer holder;rotating the polishing surface; introducing a slurry onto the polishingsurface, wherein the slurry includes:abrasive particles, most of whichare larger than a selected particle size; an oxidizer mixture, themixture having been filtered to remove particles so that are larger thanthe selected particle size; and a suspension agent; and; planarizing thewafer using the rotating polishing surface and the slurry.
 6. A methodof making a chemical-mechanical polishing slurry, the methodcomprising:mixing an oxidizer with a solution to produce a mixture witha dissolved oxidizer; filtering the mixture to remove a substantialamount of particles therein that exceed a selected particle size; addinga suspension agent to the mixture; and adding abrasive particles to themixture after filtering the mixture a substantial amount of the abrasiveparticles exceeding the selected particle size.
 7. The method of claim6, wherein the oxidizer is a ferric salt oxidizer.
 8. The method ofclaim 6, wherein the abrasive particles are selected from the groupconsisting of Al₂ O₃, SiC, SiO₂, CeO₂ and Si₃ N₄.
 9. The method of claim6, wherein the suspension agent is an aqueous surfactant that inhibitsgrowth of the preexisting particles.
 10. The method of claim 6, whereinthe particles in the mixture include reaction product particles formedin the mixture.
 11. The method of claim 6, including adding thesuspension agent to the mixture before filtering the mixture.
 12. Themethod of claim 6, including adding the suspension agent to the mixtureafter filtering the mixture.
 13. The method of claim 12, includingadding the suspension agent to the mixture immediately after filteringthe mixture.
 14. The method of claim 12, including pre-mixing thesuspension agent and the abrasive particles, and then adding thesuspension agent and the abrasive particles to the mixture.
 15. Themethod of claim 12, including adding the abrasive particles to themixture after adding the suspension agent to the mixture.
 16. A methodof making a chemical-mechanical polishing slurry, the methodcomprising:mixing a ferric salt oxidizer with a solution to produce amixture with a dissolved ferric salt oxidizer; filtering the mixture toremove most particles therein that exceed a selected particle size;adding a suspension agent to the mixture; and adding abrasive particlesto the mixture after filtering the mixture wherein most of the abrasiveparticles have a larger particle size than the selected particle size.17. The method of claim 16, wherein the selected particle size is atmost about 0.1 microns.
 18. The method of claim 16, wherein the selectedparticle size is about 0.1 microns.
 19. The method of claim 16, whereinthe abrasive particles are selected from the group consisting of Al₂ O₃,SiC, SiO₂, CeO₂ and Si₃ N₄.
 20. The method of claim 16, wherein theferric salt oxidizer is selected from the group consisting ofFe(NO₃)₃.9H₂ O, FeCl₃.6H₂ O, Fe₂ (SO₄)₃.5H₂ O and FeNH₄ (SO₄)₂.12H₂ O.21. The method of claim 16, wherein the suspension agent is an aqueoussurfactant.
 22. The method of claim 16, wherein the solution is water.23. The method of claim 16, wherein the particles in the mixture includereaction product particles formed by reacting the ferric salt oxidizerin the mixture with at least carbon in the mixture.
 24. The method ofclaim 16, wherein the particles in the mixture can include organic nitrocompounds.
 25. The method of claim 16, wherein the particles in themixture are mostly reaction product particles formed by reacting theferric salt oxidizer in the mixture.
 26. The method of claim 16, whereinthe particles in the mixture include undissolved ferric salt oxidizerparticles in the mixture.
 27. The method of claim 16, including addingthe suspension agent to the mixture before filtering the mixture. 28.The method of claim 16, including adding the suspension agent to themixture after filtering the mixture.
 29. The method of claim 28,including adding the suspension agent to the mixture immediately afterfiltering the mixture.
 30. The method of claim 28, including pre-mixingthe suspension agent and the abrasive particles, and then adding thesuspension agent and the abrasive particles to the mixture.
 31. Themethod of claim 28, including adding the abrasive particles to themixture after adding the suspension agent to the mixture.
 32. The methodof claim 16, wherein most of the abrasive particles have a largerparticle size than that of the preexisting particles remaining in themixture after filtering the mixture.
 33. The method of claim 16, whereinmost of the particles in the mixture are reaction product particlesformed in the mixture.
 34. The method of claim 16, wherein the slurry isadapted for polishing a metal selected from the group consisting oftungsten, aluminum and copper.